Phase Contrast Mode Research Articles

Simulation and reconstruction of light-microscope images

Simulation of light microscope image assists in interpretation and allows comparison of different imaging techniques and their associated parameters. Further, reconstruction of specimen properties from images can in some cases be feasible.Imaging methods can be broadly divided into fluorescent and non-fluorescent techniques, the latterconsisting of brightfield transmission or reflection modes, darkfield or various phase contrast modes. The distinctive feature of these non-fluorescent modes is that they image the material properties of the specimen, including refractive index, absorption coefficient or birefringence. Sometimes the specimen is labelled with a stain, the concentration of which mirrors the concentration of a particularconstituent. In fluorescence microscopy, usually a fluorochrome is used to label particular components or constituents.In non-fluorescent microscopy, images can be simulated using rigorous diffraction methods, such asmodal analysis or coupled wave theory. Such theories take into account multiple scattering, shadowingeffects and also depletion and aberration of the light as it penetrates the object.

Micromagnetic structure of domains in Co/Pt multilayers. II. Investigations of domain structures in written marks

An analysis is presented of the detailed effects of varying the write parameters in Co/Pt multilayer films. Domains written thermomagnetically by laser modulation have been imaged using the modified differential phase contrast mode of Lorentz electron microscopy. The effects of different laser powers and bias fields were investigated. While the domain size increases with laser power, the bias field is found to have a profound effect on domain regularity. The periodicity of the irregular circumference of the domains written at high-bias fields closely resembles the mean repeat in the ac-demagnetized state suggesting that it arises as a relaxation phenomenon.

Micromagnetic structure of domains in Co/Pt multilayers. I. Investigations of wall structure

An analysis of the micromagnetic structure of domains and domain walls in Co/Pt multilayer films is reported. Magneto-optically written domains have been imaged in a scanning transmission electron microscope by using the modified differential phase contrast mode of Lorentz electron microscopy. These have been compared with computer-simulated images based on a two-dimensional model of a circular, perpendicular magnetized domain with a Bloch-like wall structure. Agreement is found for the domain and stray field contrast, but the absence of wall contrast in the experimental images indicates a more complex wall structure in the multilayer than was assumed by the model. In a further series of calculations the magnetic microstructure of a Co/Pt multilayer was modeled by solving the Landau–Lifshitz–Gilbert equations. These suggest that the wall structure varies throughout the thickness of the multilayer, allowing significant saving of magnetostatic energy through the establishment of flux closure paths close to the walls, and are consistent with experimental observations.

STEM of order and dynamics in novel magnetic materials

The new detection system of the HB5 STEM enables the operation of the microscope in a variety of modes. If the specimen is placed inside the objective lens, high resolution bright and dark field images revealing the microstructure can be obtained. Since the specimen is located in a high magnetic field, magnetic ordering phenomena are disturbed and no magnetic structure contrast is observed. In the out-of-field position, magnetic structure may be revealed in the Fresnel and Differential Phase Contrast modes of Lorentz Microscopy. Fresnel images are obtained with a static beam, i.e. the scanning coils are not excited. In the Fresnel mode the objective lens focus is changed slightly and domain walls appear in the shadow image as dark and bright lines, switching contrast when the objective lens is changed between overfocussing and underfocussing. Electrons, passing through the sample, are deflected by the Lorentz force and appear to converge towards or diverge from the domain walls.

Characterization of magnetic microstructure at high spatial resolution

Advances in metal epitaxial growth techniques have produced renewed interest in magnetic materials through the unique properties of novel atomically engineered structures. Ultrathin layered and nanocrystalline, textured composites possess unique magnetic properties which are intimately connected to the physical microstructure on both the atomic and nanometer length scales. The length scales present in magnetic interactions span several orders of magnitude from the atomic length scales of the exchange interaction, to the mesoscopic length scales important in anti-ferromagnetic RKKY coupling between thin ferromagnetic films and small particles, to the macroscopic length scales determined by the long range magnetostatic interactions which are responsible for the formation of magnetic domains.Special characterization and computation techniques are required to analyze the micromagnetic and microstructural properties of these novel, low dimensional systems. In order to correlate the structure with the magnetic properties of any system, in-situ growth, structural, chemical and magnetic characterization is desirable.Most methods used for the observation of micromagnetic structure rely on a contrast mechanism derived from the magnetic fields present in ferromagnetic systems. We have implemented the conventional Fresnel, and less conventional Differential Phase Contrast modes of imaging magnetic microstructure in an HB5 STEM.

Investigation of the micromagnetic structure of cross-tie walls in permalloy

The modified differential phase contrast (MDPC) mode of Lorentz electron microscopy was used to study the micromagnetic structure of cross-tie walls in permalloy. The distribution of magnetic induction along these walls is characterized and studied in detail by vector mappings calculated from MDPC image pairs. The wall thickness of the cross-tie wall is determined by fitting the calculated image signal of an analytical function to the MDPC wall profiles. A pronounced dip in the wall width is found at the center of the vortex structure of the cross-tie wall. The results are compared to theoretical calculations by Nakatani, Uesaka, and Hayashi, Jpn. J. Appl. Phys. 28, 2485 (1989).

An investigation of thermomagnetically written domains in Co/Pt multilayers

We have investigated thermomagnetically written marks in Co/Pt multilayers using the modified differential phase contrast mode of Lorentz microscopy. Images thus obtained allow the domain structure in individual marks to be studied with a resolution of ⋍20 nm and can be analysed quantitatively for comparison with recording data.

Resolution and optimum conditions for dark-field STEM and CTEM imaging

Optimum values for the objective aperture angle and defocus for dark-field imaging in STEM and CTEM are calculated based on the point-scatterer approximation. The effect of chromatic and spherical aberration is taken into account. The resolution limit for the dark-field mode and several phase-contrast modes is calculated. The comparison shows that some phase-contrast imaging modes (e.g. hollow-cone illumination in CTEM and the difference signal of a ring detector configuration in the STEM) are superior in resolution power to the dark-field mode.

Electrically conductive grades of carbon black: Structure and properties

Carbon structure of nine commercial conductive blacks, comprising benzene- and acetylenebased blacks was analyzed. This series was selected for its use in dry cells (Leclanché type). TEM (transmission electron microscopy) and X-ray diffraction were employed. First, it has been confirmed that the classical “crystallite” as defined by L a and L c is not the basic building unit of the black structure. This unit is seen to be the carbon layer itself as defined by its size and state by means of TEM phase contrast mode. First, the sizes of the distorted layers were seen to range at three different scales [i.e., less than 1 nm, about 5 nm or over (few tens and over)]. Then taking into account the distortions as a second parameter, five different types were discriminated, say five different turbostratic stacking orders. Type I is defined by a layer size of 0.8 nm and a coherent stack of three layers. In type II, layer size is larger: 1 nm in average and perfect enough to stack by four. Type III has the largest layer size compatible with a concentric texture of the particle: about 5-nm large. The number of layers stacked together is ten in average. Type IV has distorted layers over 5 nm in diameter and stacked by 5 6 , whereas type V has the largest layer sizes (few tens) often straight and stacked by 7 8 . Those largest layer sizes exist exclusively with the “hollow-shell” morphology. Following this model, all the productions were actually characterized by one or few populations of aggregates with a typical turbostratic stacking order and related to properties.

Confocal and differential phase imaging of thin organic films

SUMMARY This paper discusses the imaging of thin organic films in scanning optical microscopes using both differential phase contrast and confocal modes. The Lang-muir-Blodgett technique is used to deposit thin films of controllable thickness. Step structures in these films are considered and theoretical models of the imaging are compared with experimental data. The model provides a measurement of film parameters such as thickness and permittivity. The differential phase contrast mode is also proposed as a simple method of assessment of film quality.

Investigation of domain structures in Co/Pt multilayers by modified differential phase contrast microscopy

Domain structures in multilayers of Co/Pt have been imaged in a scanning transmission electron microscope using the modified differential phase contrast mode of Lorentz microscopy. Observations have been made of films in their ac-demagnetised state and, through in-situ field application, in a series of remanent states. From the former experiments characteristic lengths related to the domain wall energy have been determined whilst the latter give insight into the magnetisation processes within the multilayers. In addition to providing data on the switching fields, direct information is obtained concerning domain nucleation and the development of domain structure approaching saturation.

Microroughness Measurements on Polished Silicon Wafers

Six different techniques i.e. Stylus profilometry, Phase Shift Interferometry (PSI), Scanning Optical Microscopey in Differential Phase Contrast mode (SOM-DPC), Light Scattering Topography (LST), Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have been used for measurements of silicon surface microroughness. Specimens with 4 different surface roughness levels have been prepared by variation of the mechanical or chemical components of a traditional polishing process. At low microroughness, a contradiction is observed between 2 groups: on one hand stylus and PSI, which sample the longer length scales from about 1 micron upwards, and STM and AFM on the other hand, which in this work sampled in the submicron range. The results of SOM-DPC and LST, which sample intermediate roughness scales, coincide with the latter group.

Optical imaging of microroughness on polished silicon wafers

The microscopic surface morphology of device quality polished silicon wafers has been imaged using laser scanning optical microscopy (SOM) in differential phase contrast (DPC) mode. The SOM-DPC technique has subnanometre vertical sensitivity and submicron lateral resolution. Surface texture was observed on all wafers examined. This was found to depend mainly on the originating wafer supplier and year of manufacture, rather than on any characteristic of the silicon material itself, and was assigned to silicon surface roughness. The amplitude of the directional 1–10 μm wide polishing features observed on many of the as-received wafers was estimated to be in the region of 1–2 nm. An additional isotropic submicron-scale roughness (incompletely resolved optically) was present in all cases. Atomic misorientation steps were also observed on specially-prepared wafers.

Mapping induction distributions by transmission electron microscopy (invited)

The Foucault and differential phase contrast modes of transmission electron microscopy are described and their uses for mapping induction distributions in magnetic thin films compared. The former is simpler to implement but the latter can provide quantitative information to much higher spatial resolution. Examples are given of instances where each is most suitable. Foucault images recorded during in situ magnetizing experiments provide valuable information on the magnetization process in small regular particles while the detailed magnetization distribution close to walls in soft magnetic films is best investigated using modified differential phase contrast microscopy.

Micromagnetics of Co-based media: Experiment and model (invited)

The micromagnetics of Co86Cr14 thin-film longitudinal recording media has been investigated using Lorentz transmission electron microscopy, in the Fresnel and differential phase contrast (DPC) modes. The media studied were 350-Å-thick Co86Cr14 on a 200-Å Cr layer, both of which were rf sputtered onto Si3N4 membranes for ease of observation in the TEM. Different magnetization states were investigated by imaging the sample at various points in the remanent hysteresis loop. This allowed direct comparison between experimental conditions and a micromagnetic computer model which assumes the media to be an array of single-domain particles, interacting via magnetostatics and moderate integranular exchange [J.-G. Zhu and H. N. Bertram, J. Appl. Phys. 63, 3248 (1988)]. The DPC imaging technique allows vector maps and two-dimensional histograms of the integrated in-plane magnetic induction to be obtained at each magnetization state for comparison with the computer model. The scale and displacement of magnetization vortices and ripple between different states was also investigated. Both experiment and model show increasing dispersion with increasing reversal field and motion of magnetization vortices. Comparison, using the model, of individual magnetization states in an applied field and at remanence indicate a strong influence of stray fields from the media on the DPC image contrast.

Impact of Various Polysilicon Deposition Process on Thin Gate-Oxide Properties in Submicron CMOS Technology

AbstractThe dielectric quality (defect density, Do and breakdown strength, Fbd) of 150Å SiO2 gate oxide (GOX) films grown by conventional or stacked oxidation scheme are discussed from the leakage measurements of polysilicon capacitors on test structure simulating our submicron CMOS process. Various polysilicon (poly) deposition processes from silane pyrolysis (570°C -620°C) were used by the low pressure chemical vapor deposition (LPCVD) technique. Both in situ and ex situ poly doping by phosphorus (P) were used to ascertain their impact on the GOX properties. The substructural characteristics of the poly/SiO2 and SiO2/Si interfaces generated by various combinations of GOX and poly deposition processes were done by the high resolution TEM lattice fringe technique under phase contrast mode.

Measurement of domain wall widths in Permalloy using differential phase contrast imaging in stem

Domain wall width measurements have been made on small regularly shaped thin film particles of Permalloy using the differential phase contrast (DPC) mode of Lorentz microscopy. The results obtained show that walls are well represented by a one dimensional symmetrical variation of the magnetisation across the wall consistent with the model of Néel.

Modified differential phase contrast Lorentz microscopy for improved imaging of magnetic structures

A modification to the differential phase contrast mode of Lorentz microscopy is proposed in which an annular quadrant detector is used instead of the standard solid quadrant detector. The new imaging mode allows a high degree of control over the relative efficiencies with which low and high spatial frequency information can be transferred from the specimen to the image. This makes it possible to effect a considerable separation of contrast arising from magnetic and nonmagnetic origins in thin polycrystalline magnetic films and thus reveals finer magnetic detail than was hitherto possible. Experimental details of the implementation of the technique, together with examples of its use for investigating thin-film recording media, are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-resolution imaging of defects in III-V compound wafers by near-infra-red phase contrast microscopy

The development of a high-resolution infra-red microscope operating in the phase contrast mode is reported for observing defects in III-V compound wafers. Strings of microprecipitates smaller than 1 µm in diameter connected by arched dislocation paths have been observed in indiumdoped GaAs. These results give similar patterns to those previously seen by other workers using A/B etching.

Magnetic structure determination in small regularly shaped particle using transmission electron microscopy

The magnetic domain structures supported by regularly shaped permalloy (Ni/sub 82.5/Fe/sub 17.5/) particles whose in-plane dimensions are in the micrometer and submicrometer range are investigated. Two modes of Lorentz microscopy are used in this study; the Fresnel mode of conventional transmission electron microscopy and the differential phase contrast mode of scanning transmission electron microscopy. Results from particles of varying size and shape are presented and the transition from multidomain to near-single-domain behavior is discussed. Finally, some details are given of how the domain-wall structure varies with particle thickness and the angle through which the magnetization rotates.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>